Monte72Carlo Posted February 18, 2009 Share Posted February 18, 2009 I have found a complete bent kit that someone is selling and am wondering if 3.5" is just too big for a small block.... the only reason I'm asking is that it's a descent price and would save me some money. I found this:http://www.hotrod.com/projectbuild/113_0411_flowmaster_exhaust_system/index.html It shows that size isn't a factor as far as going larger. Thanks, Garrett. Quote Link to comment Share on other sites More sharing options...
Shadow Gray SS Posted February 18, 2009 Share Posted February 18, 2009 Thanks for the link . Interesting & contrary to what I have heard before. Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted February 18, 2009 Share Posted February 18, 2009 right in that article it states... Quote: Last month we also proved that running open headers on this engine without collector extensions produced less power, and comparing the results of both rounds of testing shows that the full 3-inch exhaust system was on par with the open headers, which backs up our contention that running open headered (at least without collector extensions) is probably costing you power. this also holds true to a over-sized exhaust system, and if you noticed, they don't talk about the bottom end, too large of a exhaust will also kill torque on the low end, yes I have a 3 1/2" exhaust BUT!!! I have a 4000 RPM converter too (plan to get a 5000) so the bottom end isn't a factor for me Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted February 18, 2009 Share Posted February 18, 2009 yes I do, I am tuning it to race with the exhaust on, not off, then I can run a street tire class too Quote Link to comment Share on other sites More sharing options...
kc8oye Posted February 18, 2009 Share Posted February 18, 2009 yep.. too big of an exhaust, and you loose velocity.. when you loose velocity, you loose the scavenging effect.. when you loose that.. you loose power. I read somewhere (one of the various chevy magazines I'm sure) that they were suggesting that as you progress down the length of the exhaust you should gradually decrese pipe size to maintain gas velocity.. so start with 3" collectors.. down to 2.5" pipe.. into the mufflers, and 2.25 out of the mufflers and even down to 2.0" near the tail pipes kind of thing.. the same applies to header primary tubes too. too big, and you loose low end torque.. too small, and you give up high end power. it's all about picking the right combination of parts ~ Quote Link to comment Share on other sites More sharing options...
monte70car Posted February 18, 2009 Share Posted February 18, 2009 I've waited to post on this thread my understanding is you want one size exhaust from the header back. And that is the way exhaust kits comes. I do not see where some one would pick up anything by going from 3" off the header to 2.5" to 2.25". I have a parts book at home that breaks down what size exhaust to run base on motor size or base on hp. I'm a big fan of 2.5" as its used the most. Best thing to do is check with local speed shops. Quote Link to comment Share on other sites More sharing options...
Jared Richey Posted February 19, 2009 Share Posted February 19, 2009 I'm going 3" from the Header Reducer to the Muffler then 2 1/2 out the back to give me enough back pressure for the street. When I go racing she'll be all open header baby. Quote Link to comment Share on other sites More sharing options...
502ci Posted February 19, 2009 Share Posted February 19, 2009 I'm going 3" from the Header Reducer to the Muffler then 2 1/2 out the back to give me enough back pressure for the street. When I go racing she'll be all open header baby. Are you saying you're running 3" to the muffler than dropping down to 2 1/2" after the muffler? Or are you reducing right after the collector from 3" to 2 1/2" then to the muffler? If it is the second one or even the first one for that matter I would suggest a Y-cutout, you can always downsize after it. Much easier to uncap at the track then unhook the whole exhaust and moving it to the side... Bill Quote Link to comment Share on other sites More sharing options...
Heff Posted February 19, 2009 Share Posted February 19, 2009 My wife told me that "size isn't a factor as far as going larger." , but I think she's just being nice! Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted February 19, 2009 Share Posted February 19, 2009 My wife told me that "size isn't a factor as far as going larger." , but I think she's just being nice! Quote Link to comment Share on other sites More sharing options...
kc8oye Posted February 19, 2009 Share Posted February 19, 2009 it's simple rob.. as the gas cools, it's VOLUME decreases..so you need to decrease the diameter of the pipe to maintain the VELOCITY of the gases. reduce from 3" collectors to 2.5" pipe.. to the mufflers.. come out of the mufflers with 2.25 pipe.. and if you have long tail pipes (like the monte) drop to 2.0" somewhere along the way too.. this actully is the setup I have on the monte, but I run 2.25 all the way out to the bumper from the mufflers... that's what the article said.. I dunno.. seems like at WOT the gas will moving thru the pipes with such velocity that it's not gonna have much time to cool before it exits the pipes at the back... Quote Link to comment Share on other sites More sharing options...
monte70car Posted February 19, 2009 Share Posted February 19, 2009 Tim please post the article that talks about stepping the exhaust down. with a free flowing you want it to either be the same size all the way though or on size bigger. the free flowing kits from flowmaster come either 2.5 to 3" from the header back. i remember seeing an article that talked about going from a 2.25" to 2.5" for air flow but not the other way. Quote Link to comment Share on other sites More sharing options...
Jared Richey Posted February 19, 2009 Share Posted February 19, 2009 Hey Bill I'll probably go with a Y-cut out. I had done thought about it. Quote Link to comment Share on other sites More sharing options...
Dans '70 Z20 Posted February 19, 2009 Share Posted February 19, 2009 Here is the article on the comparison of various step downs with x-pipe compared to open headers. http://www.carcraft.com/techarticles/exhaust_system_installation/index.html Very interesting article. Dan Quote Link to comment Share on other sites More sharing options...
kc8oye Posted February 20, 2009 Share Posted February 20, 2009 sorry rob.. I read it in a magazine somewhere a long time ago.. I have no idea which one it was. most people don't understand backpressure. and assume it's always bad... _SOME_ backpressure is neccessary. but it depends on what you want too.. if you are looking to make low end torque like say a 4x4 rock crawler, then the stepped-down exhaust system is problaby a great way to go to maximize exhaust scavenging.. on the converse.. if you have a big block screaming to 9000 rpm then you are problaby going to want to run a pair of sewer pipes under your car.. :> (right sam?) Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted February 20, 2009 Share Posted February 20, 2009 WHAT??? Quote Link to comment Share on other sites More sharing options...
Chevyss Posted February 20, 2009 Share Posted February 20, 2009 Bigger is better with a BB. 3" all the way top the bumper. Thanks to Sam and Bill. Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted February 20, 2009 Share Posted February 20, 2009 what mufflers are going with??? Quote Link to comment Share on other sites More sharing options...
Chevyss Posted February 20, 2009 Share Posted February 20, 2009 Delta Force 40 series. It's just the right sound and seems to really pull at higher rpms. Quote Link to comment Share on other sites More sharing options...
Heckeng Posted February 20, 2009 Share Posted February 20, 2009 I think that you don't necessarily need backpressure but you do need scavenging for the best performance. Quote Link to comment Share on other sites More sharing options...
kc8oye Posted February 20, 2009 Share Posted February 20, 2009 you need the back pressure to generate the velocity which creates the scavenging effect. but obviously, you reach a point where the pipe is so small that the increase in velocity is offset by the restriction of volume edit: If you have ever tried to squeeze power out of a STOCK GM TPI engine, you are very familiar with this the TPI intake runners are long and narrow which makes for great low/mid range torque.. but they fall off FAST as you get up into the higher rev's Quote Link to comment Share on other sites More sharing options...
wallaby Posted February 20, 2009 Share Posted February 20, 2009 Check this from Header Design web page: 350/454 system examples Quote Link to comment Share on other sites More sharing options...
Heckeng Posted February 21, 2009 Share Posted February 21, 2009 cross sectional area creates the velocity, depending on the size of the engine. You don't want to add artificial restriction to increase backpressure, you'll kill the performance. Otherwise we'd all be putting on stock style mufflers instead of higher performance/flowing mufflers. Quote Link to comment Share on other sites More sharing options...
kc8oye Posted February 21, 2009 Share Posted February 21, 2009 scott. aye.. stock systems are very restrictive.. but running a full open header system can hurt you too.. Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted February 21, 2009 Share Posted February 21, 2009 you can't get the perfect exhaust...what a engine wants at full throttle, and what it wants cruising down the road are two different things Quote Link to comment Share on other sites More sharing options...
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